Caption: Cardiac heartbeat cycle. Labelled animation showing the cardiac cycle of the beating heart, from systole to diastole. The animation starts with a view of a sectioned human heart, showing the heart's internal chambers, the right and left ventricle and the right and left atrium. Blood is shown entering the heart (coloured arrows): deoxygenated blood (blue) from the body and oxygenated blood (red) from the lungs. During atrial systole, the cardiac muscles in the walls of the atria contract to pump blood from the atrium on one side of the heart downwards to the ventricle underneath. This is followed by ventricular systole where the ventricles contract to pump blood around the body and to the lungs. The period of relaxation, where the heart's atria fill up with blood again to restart the cycle, is known as diastole. Unlabelled version: K005/6428. Alpha channel versions: K005/6425 and K005/6424.